The invention relates to methods and compositions for treating subterranean formations. More particularly, it relates to methods and compositions for treating a subterranean formation penetrated by a wellbore into which a gel with a high viscosity is injected. This invention specifically relates to a method and composition for reducing the viscosity of the gel upon completion of the well treatment.
Viscous well treatment fluids are commonly used in the drilling, completion, and treatment of subterranean formations penetrated by wellbores. A viscous well treatment fluid is generally composed of a polysaccharide or synthetic polymer in an aqueous solution which is crosslinked by an organometallic compound. Examples of well treatments in which metal-crosslinked polymers are used are hydraulic fracturing, gravel packing operations, water blocking, and other well completion operations.
Hydraulic fracturing techniques are widely employed to enhance oil and gas production from subterranean formations. During hydraulic fracturing, fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. As the fracture widens to a suitable width during the course of the treatment, a propping agent is then also added to the fluid. The treatment design generally requires the fluid to reach a maximum viscosity as it enters the fracture which affects the fracture length and width. The viscosity of most fracturing fluids is generated from water-soluble polysaccharides, such as galactomannans or cellulose derivatives. Employing crosslinking agents, such as borate, titanate, or zirconium ions, can further increase the viscosity. The gelled fluid may be accompanied by a propping agent (i.e., proppant) which results in placement of the proppant within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
In order for the treatment to be successful, it is preferred that the fluid viscosity eventually diminish to levels approaching that of water after the proppant is placed. This allows a portion of the treating fluid to be recovered without producing excessive amounts of proppant after the well is opened and returned to production. The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a lower value such that it flows naturally from the formation under the influence of formation fluids. This viscosity reduction or conversion is referred to as xe2x80x9cbreakingxe2x80x9d and can be accomplished by incorporating chemical agents, referred to as xe2x80x9cbreakers,xe2x80x9d into the initial gel.
Certain gels of fracturing fluids, such as those based upon guar polymers, undergo a natural break without the intervention of a breaking agent. However, the breaking time for such gelled fluids generally is excessive and impractical, being somewhere in the range from greater than 24 hours to in excess of weeks, months, or years depending on reservoir conditions. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are usually incorporated into the gel and become a part of the gel itself. Typically, these agents are either oxidants or enzymes which operate to degrade the polymeric gel structure. Most degradation or xe2x80x9cbreakingxe2x80x9d is caused by oxidizing agents, such as persulfate salts (used either as is or encapsulated), chromous salts, organic peroxides or alkaline earth or zinc peroxide salts, or by enzymes.
In addition to the importance of providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid and to resume production, the timing of the break is also of great importance. Gels which break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also lead to a premature reduction in the fluid viscosity, resulting in a less than desirable fracture width in the formation causing excessive injection pressures and premature termination of the treatment.
On the other hand, gelled fluids which break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids and severely impair anticipated hydrocarbon production. Additional problems may occur, such as the tendency of proppant to become dislodged from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation. Preferably, the fracturing gel should begin to break when the pumping operations are concluded. For practical purposes, the gel preferably should be completely broken within about 24 hours after completion of the fracturing treatment.
U.S. Pat. No. 3,960,736 (issued Jun. 1, 1976) suggests the use of acetal esters and polysaccharides in well treatment compositions. The acetal esters hydrolyze to release the component alcohols and acids, which subsequently catalyze breakdown of the polysaccharides. This breakdown reduces the viscosity of the composition.
U.S. Pat. No. 5,224,546 (issued Jun. 6, 1993) offers the use of an esterified carboxylated chelator for the hydrolysis of metal crosslinked polymer gels used in oil and gas well applications. At elevated temperatures, the esterified chelator undergoes hydrolysis to form an acid and an active ligand which subsequently removes the crosslinking metal ion from the gel and hydrolyses the polymer. This reduces the viscosity of the gel and allows removal of the material after treatment of the well.
For the foregoing reasons, there is a continuing need for a well treatment fluid which could maintain a relatively high viscosity while it is injected into a wellbore. After a sufficient period of time to allow complete well treatment, the viscosity of the fluid should decrease to a level such that the fluid could be removed relatively easily.
Most fluid breakers either reduce the fluid viscosity too soon or, on the other extreme, provide incomplete viscosity reduction. It has been discovered that a synergistic effect occurs between certain inorganic oxidizing agents and certain organic esters. Particularly, oxidizing salts having alkaline earth or transition metal cations and polycarboxylic esters are the most suitable mixtures. This combination of chemicals has been found to provide initial high viscosity fluids while also providing complete fluid degradation at later times. This combination allows for optimum fracture growth and proppant placement while enhancing the amount of treating fluid recovered after the well is placed back on production. Ultimately, this effect significantly improves well productivity.